Fixed schemas without properties failing to generate models

A Schema defined as:

```
example:
  type: object
```

should generate a Model named `example` with no `__slots__`.

This change allows generation of a model as described, and
closes #70.

openapi3/schemas.py

@@ -128,10 +128,14 @@ def get_type(self):
  # this is defined in ObjectBase.__init__ as all slots are
  if self._model_type is None: # pylint: disable=access-member-before-definition
  type_name = self.title or self.path[-1]
+ # if there are no defined properites for this model, use an empty dict
+ # to allow the model to be set up correctly
+ model_properties = self.properties or {}
+
  self._model_type = type(
  type_name,
  (Model,),
- {"__slots__": self.properties.keys()}, # pylint: disable=attribute-defined-outside-init
+ {"__slots__": model_properties.keys()}, # pylint: disable=attribute-defined-outside-init
  )
 
  return self._model_type